Objectives: (1) To develop a system for determining the role and fate of known transplanted single cell(s) (at a definite stage of differentiation and/or treatment) in the development of host blastocysts (rabbits and mice); (2) to determine the type and degree of cellular interactions in the resultant chimaeras; and (3) to investigate the effects of selected treatment of preimplantation embryos and/or selected cells, which are to be transplanted, upon subsequent embryonic development.